A four-port quadrature hybrid can be used to combine two input signals having a 90° phase difference into a single output (quadrature combiner). Conversely, it can be used to split an input signal into two output signals with a 90° phase difference (quadrature divider). For the quadrature combiner, ideally the two input ports are isolated from one another, and any reflected energy from the load is terminated in a fourth, uncoupled port. Thus, assuming input signals are in quadrature (i.e., have a 90° phase difference) and are of equal magnitude, the four-port quadrature hybrid will provide a desirable match at the output port.
A branchline circuit is a type of four-port quadrature hybrid that is capable of providing an impedance transformation. A branchline circuit may include two parallel transmission lines and two shunt transmission lines. Each of the transmission lines may be replaced with its lumped element equivalent. A branchline divider, with inductors having a Q-factor of 20 and capacitors having an equivalent series resistance of 0.20 ohms, which are typical values for elements of a gallium arsenide die, may transform a 40 ohm input resistance to two 13.9 ohm output resistances with branchline losses of approximately 1.5 dB, due mostly to dissipation.
A ladder circuit is another type of four-port quadrature hybrid that was developed to eliminate the need for throughhole silicon vias, which are required for the lumped-element equivalent circuits in a branchline combiner. While the ladder circuit performs a quadrature combining/dividing with less insertion loss as compared to the branchline circuit (approximately 1 dB less), it does not provide any impedance transformation.